The Tagalog word ewan is a conversational form of aywan. It means 'to not know.'

Ewan! = Aywan!
Dunno. = I don't know.

Ewan ko.
I don't know.

Ewan ko po.
I don't know. (addressing someone older)

Ewan ko kung anong ginawa niya.
I don't know what he/she did. 

Ewan ko ba!
Gosh, I dunno. 

Ewan ko kung bakit.
I don't know why.

The Tagalog word ewan is also used for other people too but it is not as common.

Ewan niya.
= Hindi niya alam.
He/She doesn't know.

The Visayan equivalent of this Tagalog word is Ambot.